IE 6 & 7, Nice Menu drop down hidden behind content

This is a common problem, and I recently found a simple solution for it: http://webdemar.com/webdesign/superfish-jquery-menu-ie-z-index-bug/

While that site talks about the Superfish jQuery menu, the same solution applies to Nice Menus.
You simply need to specify that the div containing the menu has a higher z-index than the div immediately below it in the DOM that contains the content the menu hides behind.

In your case, this equates to:

#header {
  z-index: 2;
}

#allContent {
  z-index: 1;
}

I tested this on your site in IE7 and it worked, and though I couldn't test in IE6 I imagine it'll work there too.